P5.7 Step B1: remove the compiler_call IR op + the hook Registry

The compiler_call op + #compiler hook mechanism was fully superseded by
abi(.compiler) VM-native dispatch (P5.5) — no sx code emits it anymore.

Remove: the compiler_call op variant + CompilerCall struct (inst.zig); the
Builder.compilerCall emitter (module.zig); the two dead producer blocks in
lower/call.zig (compiler_expr-bodied free fns + methods); every consumer
switch arm (emit_llvm, ops.emitCompilerCall, print, interp dispatch); the
interp.hooks field + init/deinit. Strip compiler_hooks.zig down to the still-
live BuildConfig / BuildHooks / AssetDir (delete HookError/HookFn/Registry/
registerDefaults + all hookXxx, and the now-unused interp/Value imports).

Test refs that used compiler_call as a sample unported op now use vec_splat.

501/501 unit + 706/0 corpus.

src/ir/inst.zig

@@ -207,7 +207,6 @@ pub const Op = union(enum) {
     call_indirect: CallIndirect,
     call_closure: CallIndirect,
     call_builtin: BuiltinCall,
-    compiler_call: CompilerCall,
 
     /// `#objc_call(ReturnT)(recv, sel, args...)` — dispatched through
     /// `objc_msgSend`. emit_llvm.zig synthesizes a per-call-site LLVM
@@ -473,11 +472,6 @@ pub const BuiltinId = enum(u16) {
     type_info,
 };
 
-pub const CompilerCall = struct {
-    name: u32, // StringPool id for qualified name (e.g. "BuildOptions.add_link_flag")
-    args: []const Ref,
-};
-
 pub const ClosureCreate = struct {
     func: FuncId, // trampoline function
     env: Ref, // allocated env pointer (or Ref.none for no captures)